The present invention relates to subminiaturised adjustable capacitors. It relates, more particularly, to the kind of capacitor in which the capacitance value can be increased or reduced during adjustment, the final adjusted value being fixed.
Within the field of capacitor types, the present invention is concerned with the category of low-capacitance adjustable capacitors generally used for example for the fine adjustment of the precise frequency of oscillation of an oscillator or of a tuned circuit belonging to an amplifier or a filter.
DESCRIPTION OF PRIOR ART
This category of capacitors comprises in particular trimming and padding capacitors whose rated C.C. voltages do not generally exceed 250 volts and whose capacitance is encountered within a range extending roughly between around 1 picofarad and 500 pF. These capacitors fall into the following kinds:
The "screw" type in which one springy member formed by a flat, conductive, elastic electrode is positioned by means of an insulated adjuster screw, in relation to one face of another fixed, flat, conductive electrode, insulated from the first by an intervening dielectric; the capacitance of the capacitor is adjustable between about 0.5 and 15 pF; PA0 the "tubular" type in which one electrode is constituted by an external metallized surface on an element whose penetration into a tube forming a dielectric wall and having an external metallized surface to form the fixed electrode, is variable; the capacitance seldom exceeds 60 pF the minimum value being around 10 % less than the maximum; PA0 The "plate" type in which around half the surface of one ceramic disc is metallized to form an electrode; the maximum capacitance can reach around 500 pF and the minimum is about 20 % less than the maximum, adjustment being effected by rotation of one plate on another, with corresponding superimposition of the electrodes.
In addition, mention should be made of the differential type of capacitor and the capacitor which is formed using two twisted, insulated wires.
It should be borne in mind that the design of the plate type ceramic capacitor which offers a substantial capacitance variation and a high maximum capacitance, presents greater difficulties when its dimensions are reduced. In other words, in this case the superimposable surfaces of the capacitor electrodes occupy around half of the plate, the capacitance adjustment can only be carried out with an accuracy corresponding to half a turn of the control spindle and the dielectric is extremely thin and has to present a flat finish in order to prevent the occurence of corrugated spots, thus permitting to increase the capacitance and avoiding the formation of an air layer of irregular thickness which could act as a dielectric between the electrodes.
When the device to which the variable capacitor is to be fitted is relatively very small, for example an electronic watch, where the space available for the arrangement of the capacitor is very restricted, then its design presents problems.